Goodwill and Intangible Assets - Schedule of Changes in Balances (Detail) $ in Millions	9 Months Ended
	Jun. 30, 2025 USD ($)
Goodwill And Intangible Assets [Line Items]
Beginning balance	$ 133
Foreign currency impact	0
Ending balance	133
Reinforcement Materials [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	48
Foreign currency impact	1
Ending balance	49
Performance Chemicals [Member]
Goodwill And Intangible Assets [Line Items]
Beginning balance	85
Foreign currency impact	(1)
Ending balance	$ 84